FIG. 1 is a schematic circuit diagram showing a conventional switching power supply circuit. Generally, the circuit as shown in FIG. 1 has been used as a switching power supply circuit. However, the conventional switching power supply circuit has a problem in that a switching element 21 makes a great switching loss due to a recovery current developed when a diode 24 is in the off state.
Here, the recovery current represents a current which flows in the moment that a reverse-bias voltage is applied to the diode.
In recent years, it has been required to increase the efficiency of power supply equipment to reduce energy consumption. In order to meet this request, there has been proposed a step-up switching power supply circuit capable of reducing the switching losses of switching elements in Japanese Patent Application laid open No. HEI6-311738 (reference 1). FIG. 2 is a schematic circuit diagram showing the conventional step-up switching power supply circuit disclosed in reference 1. FIG. 3 is a timing chart for switching elements 21 and 28 shown in FIG. 2.
In the circuit as shown in FIG. 2, the switching element 28 is turned on just before the switching element 21 is turned on. Besides, immediately after the switching element 21 is turned on, the switching element 28 is turned off.
When the switching element 28 goes into the on state first, the current that is passed through the switching element 28 rises moderately because of a choking coil 30. Consequently, switching losses, which occur when the switching element 28 goes into the on state from the off state, can be reduced.
When the current passing through the choking coil 30 increases so as to be equal to the current passing through a choking coil 20, a capacitor 23 that is connected in parallel with the switching element 21 loses its electric charge due to the resonance (resonant circuit) of the choking coil 30 and the capacitor 23. After the capacitor 23 has finished the discharge, a diode 22 that is connected in parallel with the switching element 21 is switched on. The switching element 21 is turned on while the diode 22 is in the on state, that is, the switching element 21 is switched on when the voltage has fallen to zero (zero-voltage switching). Thus, switching losses can be reduced.
On the other hand, when the switching element 28 goes into the off state from the on state, the voltage rises rapidly across the switching element 28 due to the energy stored in the choking coil 30. Consequently, the switching element 28 incurs switching losses.
The energy stored in the choking coil 30 is discharged through a current pass of the switching element 21, the choking coil 30 and the diode 32.
As described above, the switching power supply circuit shown in FIG. 13 has a problem in that switching losses occur when the switching element 28 is turned off.
There is found other prior art aiming at reducing switching losses and improving efficiency in a switching power supply.
Japanese Patent Application laid open No. 2001-197740 (reference 2) and Japanese Patent Application laid open No. 2002-262570 (reference 3) have disclosed switching power supply equipment, respectively, with a view to enhancing the efficiency of a low-loaded low-loss switching circuit in which capacitors and switching elements are added to the main transformer of a self-excitation flyback converter.
Japanese Patent Application laid open No. 2002-112544 (reference 4) has also disclosed switching power supply equipment, in which, however, a switching power supply circuit has a different input power supply connections than that in the switching power supply equipment of references 2 and 3.
As can be seen in the aforementioned references, in order to reduce switching losses, it is necessary to suppress the rise of the voltage across the switching element. Besides, zero-voltage switching should be performed.